Ceramic materials are of critical importance for a number of high temperature, high performance applications such as gas turbines. These applications require a unique combination of properties such as high specific strength, high temperature mechanical property retention, low thermal and electrical conductivity, hardness and wear resistance, and chemical inertness. Design reliability and the need for economical fabrication of complex shapes, however, have prevented ceramic materials from fulfilling their potential in these critical high temperature, high performance applications.
The design reliability problems with ceramics, and the resultant failure under stress, are due largely to the relatively brittle nature of ceramics. This, in combination with the high cost of fabricating complex shapes, has limited the usage of ceramics.
Ceramics made from organosilicon polymers have the potential to overcome these problems. To this end, polymers based on silicon, carbon and/or nitrogen and oxygen have been developed. See, for example, "Siloxanes, Silanes and Silazanes in the Preparation of Ceramics and Glasses" by Wills et al, and "Special Heat-Resisting Materials from Organometallic Polymers" by Yajima, in Ceramic Bulletin, Vol. 62, No. 8, pp. 893-915 (1983), and the references cited therein.
The major and most critical application for ceramics based on polymer processing is high strength, high modulus, reinforcing fibers. Such fibers are spun from organosilicon preceramic polymers and are subsequently converted to ceramic materials, in particular, silicon carbide/silicon nitride bearing fibers by a two-step process of curing to render the preceramic polymeric fibers insoluble followed by pyrolyzation comprising heating the fiber in an inert atmosphere up to about 2,000.degree. C. whereupon the fibers are converted to ceramic form.
U.S. Pat. No. 3,853,567 is an early example of thermally treating a polysilazane resin to form ceramic articles comprising silicon carbide and/or silicon nitride. Thus, in Example 1 of the patent, a carbosilazane resin is formed, spun into filaments, the filaments rendered infusible by treating them with moist air for 20 hours at 110.degree. C. and subsequently heated over the course of 7 hours to 1,200.degree. C. in a nitrogen atmosphere and then to 1,500.degree. C. over the course of 5 minutes. A black-glistening filament which is completely insensitive to oxidation at 1,200.degree. C. and is amorphous to x-rays is disclosed as obtained. Subsequent heating to 1,800.degree. under argon produced a fiber consisting of .beta.-SiC, a little .alpha.-SiC and .beta.-SiC.sub.3 N.sub.4.
U.S. Pat. No. 4,399,232 discloses forming continuous inorganic fibers consisting substantially of Si, Ti and C and optionally O obtained from pyrolyzing a polycarbosilane having side chains containing titanoxane units. The polycarbosilane is spun into fibers, the resultant fibers subjected to curing and the infusible fibers pyrolyzed into inorganic fibers. Free carbon in the fibers can be removed by heating the resulting inorganic fibers in an atmosphere of at least one gas selected from the group consisting of oxygen gas, air, ozone, hydrogen gas, steam and carbon monoxide gas preferably at a temperature of 800.degree. C. to 1600.degree. C.
While ceramic fibers formed from organosilicon preceramic polymers are intended to be used in high temperature environments, it has often been found that when aged at high temperatures, i.e., 1400.degree. C. for 12 hours, these ceramic fibers are crystallized to an unusable brittle material whereby the thermal stability performance, i.e., change in weight and tensile properties, of the fiber are vastly degraded from original values. Thus, if ceramic fibers formed from organosilicon preceramic polymers are to be used to the fullest potential, there is a need for improving the thermal properties of such fibers and render them fully suitable for high temperature, high performance applications.
It is a primary object of the present invention to enhance the thermal stability performance of ceramic fibers which have been formed by the pyrolysis of organosilicon preceramic polymers.
It is another object of the present invention to provide thermally stable ceramic fibers from organosilicon preceramic polymers for use in high temperature, high performance applications.
These and other objects, aspects and advantages of the invention will be readily apparent to those of ordinary skill in this art upon consideration of the following description of the invention.